XV. THE FROG AND VERTEBRATES IN 

 GENERAL 



The animal kingdom is commonly said to be divided into the 

 Invertebrates and the Vertebrates. The basic distinction be- 

 tween these two groups may be said to be the presence in the latter 

 of a dorsal supporting axis, the vertebral column, which is of para- 

 mount importance in its relations to the general supporting struc- 

 tures of the body and in the protection rendered to the spinal cord. 

 It is evident, therefore, that the several metazoan phyla so far 

 noted have consisted only of Invertebrates. The Vertebrates, 

 on the other hand, belong to one phylum, the Chordata. This 

 phylum also includes, in addition to the important vertebrate divi- 

 sion, a small number of types which, for the most part, are aberrant 

 structurally but possess, nevertheless, certain basic features which 

 seem to link all of them together. These distinctive features may 

 now be noted : 



(1) A dorsal supporting axis, the notochord, is always present 

 either throughout life or, at least, during early development. The 

 notochord is a rod-like structure which typically extends the 

 entire length of the animal, dorsal to the alimentary canal, but it is 

 subject to considerable variation in the different chordate groups. 

 In the Vertebrates, the vertebral column develops in close relation 

 to the notochord. 



(2) A well-developed, tubular central nervous system which 

 lies dorsal to the notochord and alimentary canal is present either 

 during embryonic development or throughout life. It will be 

 remembered that in the Invertebrates which possess a central 

 nervous system, the nerve cord always lies ventral to the alimen- 

 tary canal and is a solid cord, instead of a tube with a central 

 cavity as in the Chordates. 



(3) At some period in their life history the Chordates typically 

 possess paired lateral openings, situated near the anterior end of the 

 body, which connect the cavity of the pharynx directly with the 

 exterior. These openings are known as gill slits and, when 

 functional, permit the water taken in through the mouth to pass 



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